At present, these reinforcements are frequently constituted by an unwoven material coated and impregnated with bitumens modified with polymers.
This polyester reinforcement tends to replace more and more the nonwoven reinforcement constituted of glass fibers for reasons of superior resistance to puncturing and cracking, in combination with better mechanical characteristics as to rupture and elasticity.
However, this nonwoven polyester reinforcement is less dimensionably stable than nonwoven reinforcement constituted of glass fibers, which often requires their stabilization by combination with a mineral structure, such as a network of glass fibers, in the form of a grid or parallel filaments, principally to avoid deformation of these reinforcements during the application of bitumen, and eventually of the sealing sheet disposed on the roofing.
But at present, one of the most serious problems connected with such reinforcement is their flammability.
Thus, in the case of fire, it is first of all necessary to control the spread of the fire so as to avoid other immediately adjacent buildings catching fire.
But when burning material falls on the roofing sheet, even when covered with gravel or slate, the latter may catch on fire, principally because of the bituminous binder, but also because of the presence of synthetic fibers such as polyester.
This problem moreover becomes more and more serious because of the development of said reinforcement, the latter having the tendency always to contain more synthetic fibers relative to the mineral fibers. Similarly, the development of bituminous compositions renders them more flammable.
Strict rules and regulations have been enacted for several years so as to reduce the danger of the spread of fire by roofing in the case of burning buildings.
The solutions presently employed to date in sealing systems, to obtain an appropriate classification according to the rules and regulations in force against the danger of burning roofs, consist in the use of a sealing sheet, reinforced by a glass cloth, disposed in surface securement to a membrane modified bitumen reinforced with nonwoven polyester.
Other solutions to decrease the flammability of the bituminous sealing sheets have been proposed, such as for example, the addition of a fire suppressing agent in the bitumen, such as aluminum hydrate, or else the use of polyester of the type known as "flame retardant".
But these solutions are not satisfactory, because while increasing the cost of production, they do not prevent the spread of fire. Nonflammable polyester, for example, will not burn with the building, but will melt and thus contribute in spite of everything to the propagation of the fire.
One therefore seeks to create a veritable screen in the reinforcement, whose specific function is to avoid the propagation of the fire in a vertical direction and in a planar fashion.
Two types of solutions principally exist: a three-layer reinforcement, or a two-layer reinforcement.
The first type of solution, namely, the three-layer reinforcement, is not completely satisfactory, because the layer forming the anti-fire screen being always disposed between the two other layers forming the reinforcement, the layer comprising synthetic fibers, for example nonwoven polyester, will always be disposed on the surface, which arrangement is exactly to be avoided.
The second type of solution, namely, the two-layer reinforcement, has itself difficulties of production. Thus, a two-layer reinforcement must have good thermal stability, a very low shrinkage force as well as a small dimensional shrinkage, which is to say good dimensional stability.
But all the two-layer reinforcements now in existence, have either poor adhesion to the layer forming the screen on the other layer, or a low speed of impregnation during application of bitumen, or again the phenomenon of curling, particularly also during application of bitumen.
Moreover, the search for very high thermal stability often leads to unacceptably increased rigidity of the layer forming the screen, which gives rise to difficulties during handling by rolling the reinforcement.
The problem addressed by the invention therefore consists in providing a two-layer textile reinforcement adapted for the production of bituminous sealing sheets for roofing having such a fire retardant screening layer, without at the same time the presence and properties of such a layer giving rise to the various drawbacks described above.